In a self-controlled case series, the rate of venous thromboembolism (VTE) was 0.126 per person-year in the 42 days after receipt of a vaccine dose compared with 0.159 per person-year during control periods (incidence ratio 0.77, 95% CI 0.53-1.11), according to Nikolai Madrid Scheller, MB, of the Statens Serum Institut in Copenhagen, and colleagues.

The authors noted that a risk of VTE was seen in two post-licensure safety studies, "but one was based on reports from a passive surveillance system and the other included few vaccinated cases, many with known risk factors for VTE."

To explore the issue further, the researchers performed a self-controlled case series using data from Danish national registries. Of more than 1.6 million females ages 10 to 44, about one-third (31%) received the quadrivalent HPV vaccine from October 2006 through July 2013.

During the study period, there were 4,375 incident cases of VTE; 20% of them involved women who were exposed to the vaccine.

The lack of an association between vaccination and the risk of VTE the 42 days after administration remained consistent across age groups, in analyses restricted to cases receiving anticoagulant treatment and women exposed to the vaccine, and after adjustment for use of oral contraceptives, which have a known link with VTE.

"Our results ... do not provide support for an increased risk of venous thromboembolism following quadrivalent HPV vaccination," the authors wrote.

However, they added, the study was limited by potential residual confounding by unmeasured factors that can vary over time and by the possibility of invalid diagnoses of VTE.

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